A defibrillator is a device that applies electric shocks to the heart in the event of ventricular fibrillation or ventricular tachycardia.
It is generally accepted that a heart rhythm exceeding, e.g., 300 beats per minute corresponds to a ventricular fibrillation, justifying triggering of the defibrillator. It also is generally accepted that a heart rhythm of less than, e.g., 150 beats per minute does not correspond to a ventricular tachycardia, but rather to a sinus rhythm or sinus tachycardia. Between these two values, the ventricle can have a ventricular tachycardia, a supraventricular tachycardia, or a bi-tachycardia, the latter being the superimposition of a ventricular tachycardia and a supraventricular tachycardia.
It is known to divide the spectrum of cardiac frequencies into bands to define different categories of tachycardia. European patent No. 360,412 refers to a well-tolerated tachycardia as between 150 and 175 beats per minute, an averagely tolerated tachycardia as between 175 and 200 beats per minute, and a poorly tolerated tachycardia as between 200 and 275 beats per minute. The document also refers to the ventricle frequency being greater than 275 beats per minute as undergoing fibrillation, and less than 150 beats per minute as a sinus rhythm.
Different therapies are programmed for each tachycardia category as a function of criteria, namely high frequency, sudden difference of frequency, and persistently high frequency. The different therapies include non-aggressive stimulations, aggressive stimulations, cardioversion shocks, and defibrillation shocks. They are applied in isolation or in succession as a function of the category of tachycardia defined according to the heart rate observed. The different therapies may be provided by an anti-tachycardia device in a known manner. It is noted that a defibrillator is one type of anti-tachycardia device which can deliver high energy shocks (up to several joules).
The approach of EP 360,412 to the problem of ventricular tachycardia is not fully satisfactory as it does not identify the type of tachycardia occurring in the ventricle: ventricular tachycardia, supraventricular tachycardia, or bi-tachycardia.
U.S. Pat. No. 4,860,749 describes a device that picks up the signals representative of the cardiac activity in both the ventricle and the atrium. By means of an algorithm using plural frequency thresholds and comparison to delays previously introduced for each patient, the device defines different types of tachycardia. According to this U.S. patent, the atrioventricular "AV" interval during a pathological tachycardia is greater than the AV interval in the case of tachycardia with an atrioventricular association of 1:1. During a preliminary electrophysiological examination, the doctor determines a threshold value and inserts it into the device. The algorithm tests the AV intervals with regard to this threshold value in order to define the type of tachycardia encountered.
U.S. Pat. No. 4,860,749 also refers to, in the case of the ventricular rate exceeding the atrial rate, automatically concluding the presence of a ventricular tachycardia. However, it is acknowledged that this event can correspond to other situations such as: sinus tachycardia with ventricular extra-systoles, 1:1 supraventricular tachycardia with loss of atrial detection, or atrial fibrillation with a fast ventricular rate and loss of atrial detection. Moreover, the criteria chosen in this U.S. patent, which presumes the presence of a ventricular tachycardia when the AV interval exceeds the value of the AV interval in sinus rhythm, can lead to misinterpretation insofar as it is acknowledged that supraventricular tachycardias induce unforeseeable variations of the AV interval.